The catalytic epoxidation of an olefin in the presence of a silver-based catalyst producing an olefin oxide is well known in the art. Conventional silver-based catalysts have provided an olefin oxide with notoriously low selectivity. As such, when using conventional silver-based catalysts in the epoxidation of an olefin, the selectivity towards the olefin oxide, expressed as a fraction of the olefin converted, does not reach the theoretically maximal selectivity based on the stoichiometry of the reaction.
To a large extent, the selectivity determines the economical attractiveness of an epoxidation process. For example, a one percent improvement in the selectivity of the epoxidation process can reduce the yearly operating costs of a large-scale olefin oxide plant substantially.
As is known to those skilled in the art, an olefin oxide produced by epoxidation using a silver-based catalyst may be reacted with water, an alcohol or an amine to form a 1,2-diol, a 1,2-diol ether or an alkanolamine. For example, ethylene oxide may be reacted with water to form ethylene glycol which product can be used as a component of an antifreeze composition, a solvent or a base material in the production of polyethylene terephthalates. Any improvement in the selectivity of the epoxidation process can also reduce the yearly operating costs in the overall process for the production of these products.
Highly selective silver-based epoxidation catalysts have been developed which extend the selectivity to a value that is closer to the stoichiometric limit mentioned above. Such highly selective catalysts comprise a porous refractory support such as alpha alumina, which has on its surface a catalytic amount of silver and at least one promoter that improves the catalyst performance in the epoxidation process.
The use of alkali metals and transition metals as promoters for silver catalysts is well known for the production of ethylene oxide by the partial oxidation of ethylene in the vapor phase; see, for example, U.S. Pat. Nos. 4,010,155, 4,012,425, 4,123,385, 4,066,575, 4,039,561 and 4,350,616. Highly selective catalysts which contain, in addition to silver, selectivity-enhancing promoters such as rhenium, molybdenum, tungsten or nitrate- or nitrite-forming compounds, are discussed in U.S. Pat. Nos. 4,761,394 and 4,766,105. The catalyst may comprise further elements like alkali metals as described in U.S. Pat. Nos. 3,962,136 and 4,010,115.
Over the last two decades, rhenium was described as being effective in improving the selectivity of alkaline metal promoted silver-based catalyst supported by a refractory porous support; see, for example, U.S. Pat. Nos. 4,761,394 and 4,833,261. Further improvement of silver-based catalysts promoted with alkaline metals and rhenium was achieved by the use of sulfur, Mo, W, Cr as is disclosed in U.S. Pat. Nos. 4,766,105, 4,820,675 and 4,808,738.
In using highly selective silver-based epoxidation catalysts as described, a reaction modifier, for example, an organic halide, may be added to the feed for further increasing the selectivity of the process. The use of reaction modifiers is disclosed, for example, in EP-A-352850, U.S. Pat. Nos. 4,761,394 and 4,766,105.
Despite all the advances made in developing high selectivity catalysts (HSCs), these catalysts, like their conventional counterparts, still need to be conditioned during an initial operation phase of the epoxidation process. The conditioning of HSCs is required in order to ensure that the optimal reactivity of the catalyst as well as high selectivity are achieved. The conditioning process typically occurs during the start-up of the epoxidation reaction, i.e., prior to obtaining a sufficient amount of olefin oxide product.
The start-up process and hence preconditioning of epoxidation catalysts has also been described in the prior art. For example, U.S. Pat. No. 5,155,242 relates to the start-up of an epoxidation process wherein a non-HSC catalyst is subjected to a pre-soak period in the presence of the organic halide at a temperature less than the operating temperature of the reactor. U.S. Pat. No. 4,874,879 relates to the start-up of an epoxidation process wherein a HSC is subjected to a pre-soak period in the presence of the organic halide at a temperature less than the operating temperature of the reactor.
In addition to these disclosures, U.S. Pat. No. 7,102,022 discloses another start-up process for using a HSC. In accordance with this disclosure, the process includes the steps of contacting a catalyst bed comprising a silver-based highly selective epoxidation catalyst, or a precursor of the catalyst comprising the silver in cationic form, with a feed comprising oxygen at a temperature of the catalyst bed above 260° C. for a period of at most 150 hours, and subsequently decreasing the temperature of the catalyst bed to a value of at most 260° C. Another such start-up process is disclosed in U.S. Patent Application Publication No. 2004/0049061 A1 in which a supported highly selective epoxidation catalyst comprising silver in a quantity of at most 0.17 g per m2 surface area of the support is used. In accordance with this publication, the method includes contacting the catalyst, or a precursor of the catalyst comprising the silver in cationic form, with a feed comprising oxygen at a catalyst temperature above 250° C. for a duration of up to 150 hours, and subsequently decreasing the catalyst temperature to a value of at most 250° C.
None of the above disclosures provides an effective means for controlling and maintaining the start-up temperature of an epoxidation process as well as controlling the oxygen outlet concentration. As such, there is a need for providing a method in which a HSC can be conditioned at a controlled start-up temperature which exceeds the maximum achievable utilizing an external heating source, such as steam, available to the epoxidation reactor.